High-profile Florida lawyer John Rex Powell has agreed to serve at least 20 years' jail in the US for his involvement in a global pedophile ring that had a young Australian boy as its prized sex object.

The boy, as a newborn, was bought for $US8000 (NZ$9456) in Russia by Australian pedophile Peter Truong and his longtime American partner Mark Newton.

Truong and Newton lived in Cairns but they travelled the world with the boy, visiting other members of their ''Boy Lovers'' pedophile ring.

Powell, in a deal with federal prosecutors in Indiana, has agreed to serve between 20 and 30 years' jail in the US, and after his release be subject to supervision for the rest of his life, not possess pornography, participate in a programme for sexual disorders, undergo polygraph exams, register as a sex offender and consent to having his computer use monitored.

An Indiana judge will decide how much time Powell will serve.

Powell, 41, will plead guilty to a count of conspiring with Truong, Newton and others to sexually exploit the boy and a count of engaging in a child pornography enterprise.

The lawyer will also plead guilty to conspiring to sexually exploit another victim, a boy in New Jersey.

Last year in an Indianapolis court Truong, 36 was sentenced to 30 years' jail and Newton, 42, was jailed for 40 years.

The investigation began in New Zealand in 2011 after the discovery of child pornography and clues led Queensland Police to the Cairns home of Truong and Newton.

Truong and Newton were then tracked to Los Angeles, where they were arrested and the boy rescued by US authorities.

Powell went by the screen name ''thebeav'' and travelled to the Cairns home where Truong, Newton and the boy lived, prosecutors said.

Powell allegedly had sex with the boy, then aged five or six, during the stay, as well as during a trip to France.